Not the full U-turn, but you do see autos change lanes without looking from time to time, or turning across bike lanes without visually clearing them. There are idiots everywhere, it's just that the rules for operating on a roadway are more explicit, making traffic more predictable (but not, unfortunately, idiot-proof).

FWIW, I find that runners are among the most predictable of MUP users, as they tend to move in a straight line and rarely deviate. In over a decade of regular MUP use (on bike, on 'blades, on skis, and on foot), I've only once run afoul of the ol' U-turn-to-the-face maneuver from a runner (although, like you, I've seen it repeatedly); and since it was on a crowded section of path, I was going slow enough to hit the brakes and put a foot down, and watch with increasing bemusement as the runner stumbled into my front tire. I have also been run completely off of the path once, nearly run off several more times, and buzzed innumerable times by cyclists on the MUP; same kind of idiots, just with more speed.

Dude, if you blew an airhorn as you passed me on an MUP, I would have a hard time not pulling you off of the back of your bike and bellowing into your face for all that I'm worth. Save that crap for the road.